Radiant energy transducer systems find a wide range of applications in modern technology. Electrically driven transducers, for example, emit radiation to illuminate a desired area or footprint. The transducer system may illuminate the area for a number of reasons. For example, if the emitting transducer emits visible light, the illumination may facilitate use of the area by human personnel. If the illumination of the area provides infrared radiant energy, the illumination may facilitate some associated detection operation or human monitoring of the area through special night vision equipment.
Many such systems require a diffuse reflectivity on one or more surfaces within each system, and for efficiency, the reflectivity of such surfaces must be relatively high. In at least some applications, the reflective surfaces are subject to high-intensity light and attendant high temperatures.
One example of a material with a high diffuse reflectivity is SPECTRALON, and use of this material has been suggested for optical transducer systems, including light systems. Attention is directed for example to commonly assigned U.S. Pat. No. 6,064,061 to Ramer et al. This material, however, generally has to be molded and machined, and is not easily applied as a coating. Such molding and/or machining is often too expensive for mass production applications, for example, for lighting fixtures.
It has been known to use a variety of white paints for forming necessary reflector surfaces. U.S. Pat. No. 5,967,652 to Ramer et al suggested that diffusely reflective surfaces for a luminaire could be constructed of a suitable base material of, for example, aluminum or plastic, with a coating of a diffuse reflective material such as barium sulfate or quasi-diffuse reflective materials, such as white paint.
However, readily available coating materials, particularly those available in the quantities and at the prices appropriate for mass production use, have suffered from inadequate reflectivity and/or excessive sensitivity to temperature. In a luminaire, for example, if the diffuse reflectivity of a reflective surface is not high enough, then the lamp within the luminaire must emit more light to achieve a desired illumination performance characteristic. If the material providing the diffuse reflectivity is too sensitive to temperature, it can not withstand the heat from certain types of high intensity lamps. If overheated, the material may break down and become even less reflective (e.g., turn brown), or the material may even catch fire. To avoid these consequences, using existing reflective materials, luminaires often may need to be designed to provide relatively large spacings of the reflectors from the lamps, to allow for heat dissipation. This approach, however, increases the overall size, weight and complexity of the luminaire.
Exotic materials are known, which may be spray-painted under carefully controlled conditions and exhibit both a hi-reflectivity and high resistance to temperature. However, such materials have been extremely expensive, as well as difficult and expensive to apply. As a result, such materials have not been utilized in optical transducer systems, like mass produced luminaires.
For example, U.S. Pat. No. 4,111,851 to Shai discloses a coating characterized by low thermal absorption and high thermal emittance. This patented coating comprises: (a) a fired oxide pigment of aluminum oxide and zinc oxide, a vehicle-binder comprising an alkali metal silicate; and sufficient water to provide a mixture suitable for application to a substrate, such as aluminum. The patent, however, teaches use of this material for coating the surfaces of spacecraft. U.S. Pat. Nos. 5,296,285 and 5,885,658 to Bable et al. teach complicated techniques for coating metal substrates with similar white paints having low solar absorbance and high heat emittance. All three patents teach use of these materials and complex application techniques for thermal control applications on the exteriors of spacecraft.
Another example of an available inorganic white paint for such spacecraft applications is the paint marketed as Z-93 by Illinois Institute of Technology Research Institute (IITRI), which apparently comprises calcined zinc oxide particles and a potassium silicate binder. The calcination process involves firing or baking the zinc oxide, in excess of 1000° C. for greater than 8 hours, to remove impurities. Because of the time and energy required to produce calcined zinc oxide this step increases the overall cost of the material significantly, and in turn the cost of the coating. The Z-93 material currently sells for $125 per pint. Also, this material requires a very complex and expensive process to mix and apply. For example, IITRI specifies a mixing period of 6 to 8 hours in a ball end mill (a drum filled with the coating and porcelain balls). Although the material is sprayable or spot brushable, IITRI specifies very precise temperature and humidity requirements during application and curing of their paint. For example, the coating must cure at >50% for the first 5-6 hours and at ambient conditions for seven days. These requirements for preparing, applying and curing the Z-93 material radically increase the complexity and cost of manufacturing components with such a coating. While these costs may be acceptable in one-off applications, such as spacecraft, they make the use of the Z-93 coating in mass-production applications completely impractical, economically.
Hence, there is a continuing need for highly efficient radiant energy transducer systems. To support that general need, there is an attendant need for radiant energy transducer systems and for reflectors for such systems which have desirable diffuse reflective properties yet are easy and cost effective to mass produce. Hence, a need exists in the context of radiant energy transducer systems, for an easily applicable coating material, of relatively low cost, which exhibits diffuse reflectivity, is highly reflective and is relatively stable when exposed to high levels of light and heat. There is an attendant need for techniques to mix and apply such reflective materials to substrates of reflectors, in a manner that is efficient and cost effective in a mass production environment.